Twitter’s acquisition of Web security company Dasient is good news for the social network giant and its millions of users, though, on the down side, it takes the security vendor off the general market.

The acquisition is apparently focused on protecting Twitter’s growing advertising business, leveraging Dasient’s anti-malvertising service. Twitter introduced its self-serve ad service in November, and the stakes are pretty high. Research and analysis firm eMarketer reported estimated Twitter advertising revenue at $45 million in 2010 and around $139.5 million in 2011. The research company predicts revenues of about $260 million in 2012 and $400 million in 2013. (The revenue figures are estimates because Twitter is privately held.)

Dasient offers two primary security services, Website Protection and Ad Network Protection. It’s a painful fact of life that most of the websites bearing malicious content are actually compromised legit sites, rather than those that are more obviously sketchy and clearly places most sane people should avoid. Malicious advertising or malvertising has been a major security threat for some time. Automated ad purchasing makes it easy for criminals to post bogus ads, which can disseminate malware, either by sucking in gullible readers or automated JavaScripts that don’t require user interaction.

Dasient’s Web Protection service is designed to protect its clients sites against compromise and assure that they don’t disseminate malware to visitors. Besides the obvious problem of infecting unsuspecting users, companies can lose business from outraged customers and suffer brand damage. Sites can even be blacklisted, at least temporarily, if their security reputation ratings fall.

The Ad Network Protection service scans client ad networks and their sub-networks to identify malicious ads and block them.

Dasient scans client sites continually, testing them with browser and virtual machine emulation, checking against blacklist and anti-virus engines, and using behavioral analysis to attempt to detect previously unknown or zero-day exploits.

The bad news is that Dasient is closing up shop for new customers, a loss for those companies who might benefit from its services. In a blog post on its site, Dasient announced that it is “winding down its business” and is no longer accepting new customers. Dasient and Twitter say that the security company and its personnel will become part of Twitter’s revenue engineering team.

We’ve grown accustomed to consolidation in the security business, as big tech firms gobble up sharp startups with exciting technologies. The hope, in those cases, is more investment, more research resources and a wider market for security tech. Not in this case. But, consider, that as a result of the acquisition, Dasient’s web security know-how can protect a couple of hundred million Twitter users, give or take. That’s not a bad trade-off.